Effects of agonist efficacy on desensitization of phosphoinositide hydrolysis mediated by m1 and m3 muscarinic receptors expressed in Chinese hamster ovary cells

Muscarinic receptor agonist-induced desensitization of phosphoinositide (Pl) hydrolysis and loss of receptors were studied in Chinese hamster ovary (CHO) cells transfected with the m1 and m3 muscarinic receptor genes. Long-term exposure to the full agonist carbamylcholine (CBC) resulted in a time-dependent attenuation of the maximal Pl response and a decrease in agonist potency. This desensitization was accompanied by a parallel loss of maximal ligand binding without an alteration of the binding affinity. The time course of both receptor desensitization and down-regulation was similar in m1 and m3 CHO cells. The Pl response to the partial agonist McN-A-343 (McN) in m1 cells was more sensitive to desensitization by CBC than the response to the latter agonist, and this desensitization was faster than receptor down-regulation. Desensitization of the Pl response to McN was reflected as a decrease in the maximal response without a marked change in potency. McN induced slow desensitization of the Pl response to CBC but a much faster desensitization of its own response. Our data provide evidence that although muscarinic agonist-induced desensitization of Pl hydrolysis in CHO cells is due mainly to loss of receptors, there are other important factors which play a role in this process, e.g., receptor-effector uncoupling. The relative contribution of these different mechanisms depends on the efficacy of the agonists used for the receptor desensitization and activation steps.